1. Field of the Invention
The present invention relates to an electronically controlled automatic transmission.
2. Description of the Related Art
In electronically controlled automatic transmissions, various controls, including speed-change control using solenoid valves, lockup clutch control for a torque converter, line pressure control, and engine torque control, all conventionally effected by electronic control.
Regardless of whether an electronic control system or a hydraulic control system is used, a speed-change stage is determined through throttle opening and vehicle speed, and a first shift solenoid and a second shift solenoid are subjected to on-off control. At the same time, control of hydraulically controlled solenoids is determined on the basis of the throttle opening, speed-change stage, etc., so as to set line pressure for generating engaging pressure for such as speed-change clutches, brakes, and the like. In addition, consideration is given to the durability of input torque of a transmission to ensure a sufficient torque capacity, while oil pressure is set in such a manner as not to cause a feeling of slowing during speed-change.
However, with the above-described conventional electronically controlled automatic transmission, the reduction of the engine torque results in an undesirable sensation of the speed-change.
That is, since engines are generally tuned up for normal conditions of use, there are instances where, during running on highland when the temperature is high, the engine torque declines temporarily due to a drop in the air density.
In that case, since the aforementioned line pressure is set on the basis of the throttle opening which serves as a characteristic indicative of engine torque, the line pressure becomes excessively large relative to the input torque of the transmission, and speed-change shock occurs during a speed change.
The occurrence of speed-change shock will be described with reference to FIGS. 2 and 3.
FIG. 2 is a diagram illustrating relationships between time and output torque, and FIG. 3 is a diagram illustrating relationships between engine load and throttle.
As shown in FIG. 2, during running on flatland, in a case where a speed change is effected through an increase in oil pressure P supplied to hydraulic servos for such as clutches and brakes, it is possible to obtain a predetermined speed-change time, and the speed-change shock is small. However, if the engine torque declines, the line pressure becomes excessively large, and the speed-change time becomes short, with the result that speed-change shock increases by that margin.
In FIG. 3, the abscissa indicates engine load EL, while the ordinate indicates throttle opening VTH. The engine load EL is expressed percentagewise by calculating such that, if the amount of intake air introduced into the engine is AF and the engine speed is NE, then AF/NE during idling is set as 0%, and AF/NE during maximum torque is set as 100%. The drawing reveals that the engine load EL is lower in the case of a 1000-m altitude than in the case of a 0-m altitude, at the same engine speed of 3000 rpm and temperature of 25.degree. C.
Although adjustment of the line pressure is conventionally undertaken to prevent the occurrence of speed-change shocks (refer to Japanese Patent Application Laid-Open No. 1-279151), the correction of the line pressure is effected only relative to the throttle opening. With this technique, therefore, it is impossible to prevent speed-change shocks occurring due to the differences between places of running.